Routing cutter tools are employed in the machining of workpieces where it is required to remove from the workpiece substantial amounts of metal leaving precisely determined external contours, such as shoulders or recesses, in the machined workpiece. A particular example of such routing operations is in the machining of aircraft structural components such as, for example, wing structures, wherein it is required to reduce to a maximum degree the weight of the structure whilst ensuring its mechanical strength. For this purpose the workpiece, usually formed of aluminum, is subjected to compound end-milling operations including slotting and recess milling, adjacent recesses in the workpiece being very often separated by extremely thin-walled portions. It will therefore be appreciated that the routing operation involves the removal of considerable quantities of metal and production efficiency requires a relatively high rate of such metal removal. At the same time, and in view of the fact that there remain in the machined component very thin separating walls, it is vital to ensure that the routing operation involves a minimum degree of vibration, which could give rise to damage in these thin-walled portions, or induce undesirable stresses and strain therein.
Of its very nature, a routing operation involves an initial drilling or sinking penetration of the workpiece by the cutting insert in the direction of the rotary axis of the cutting tool, and a subsequent milling operation involving a relative transverse displacement of the workpiece with respect to the rotary axis of the cutter tool. It has been proposed in this connection, e.g. U.S. Pat. No. 4,946,318, to provide a rotary routing cutter tool having a cutting insert wherein the rotary axis of the cutter tool is offset with respect to the cutting insert.
A cutting insert, generally employed for this purpose, is of a substantially parallelepipedal shape, having a pair of substantially parallel major cutting edges of equal length and a pair of transversely directed, subsidiary cutting edges. Each subsidiary cutting edge merges at one end thereof with an adjacent major cutting edge via a rounded nose portion. At least a portion of each subsidiary cutting edge defines, with the adjacent major cutting edge, an acute angle. With such an insert, an axially directed movement of the tool into the workpiece, accompanied by rotation of the tool, results in the rounded nose portion and the subsidiary cutting edge of the insert effecting drilling or sinking. This is followed by a transverse displacement of the workpiece so as to route a slot in the workpiece and, to this end, the insert moves in a series of ramps as it reciprocates from one end of the slot to the other whilst being continuously displaced into the workpiece. In the course of these transverse displacements, the subsidiary cutting edge serves to remove the upstanding core of material which has been formed underneath the central portion of the cutting tool, whilst the major cutting edge serves to mill the surrounding slot wall.
In view of the fact that the rotary axis of the tool is radially offset with respect to the insert, the maximum depth of penetration of the tool into the workpiece is limited to the projected length of the subsidiary cutting edge on the adjacent major cutting edge. It will be realized that the more limited is the maximum penetration of the insert into the workpiece, the greater the number of ramps which have to be milled in order to attain the desired slot depth. Clearly, the greater the number of ramps that have to be milled, the more time consuming is the whole routing procedure.
It is an object of the present invention to provide a cutting insert for a rotary milling tool, particularly for a routing tool, wherein the above-referred to disadvantage is significantly reduced.